A pacemaker is a medical device for implant within a patient, which recognizes various arrhythmias such as an abnormally slow heart rate (bradycardia) or an abnormally fast heart rate (tachycardia) and delivers electrical pacing pulses to the heart in an effort to remedy the arrhythmias. An ICD is a device, also implantable into a patient, which additionally or alternatively recognizes atrial fibrillation (AF) or ventricular fibrillation (VF) and delivers electrical shocks to terminate fibrillation.
Pacemakers and ICDs are often provided with the capability to detect the circadian state of the patient, i.e. whether the patient is awake or asleep, and to adjust pacing parameters based on the circadian state. For example, a base pacing rate may be reduced while the patient is asleep then increased while the patient is awake. Conventionally, circadian state is detected based on time of day using an on-board clock or detected using a posture sensor. Typically, with an on-board clock, the patient is simply deemed to be awake during the day and during the evening but asleep at night. With a posture sensor, typically, the patient is deemed to be asleep while lying down. Neither technique is particularly effective. An on-board clock does not properly allow for a reduction in pacing rates if the patient sleep during the day or for an increase in pacing rates if the patient is awake at night. Posture detection does not properly distinguish between simply lying down rather than sleeping.
More sophisticated circadian state detection techniques have been developed that exploit patient activity levels detected using an activity sensor or that exploit minute ventilation detected using a thoracic impedance detector. A detailed description of an activity sensor for use in detecting circadian states is provided in U.S. Pat. No. 5,476,483, to Bomzin et al., entitled “System and Method for Modulating the Base Rate During Sleep for a Rate-responsive Cardiac Pacemaker”, which is incorporated herein by reference. Briefly, Bornzin et al. teaches the use of “activity variance” to determine if the patient is awake or sleeping. That is, an activity sensor has significantly less variability during sleep. Details of a system for exploiting minute ventilation in the detection of circadian states is set forth in U.S. Pat. No. 6,128,534 to Park et al., entitled “Implantable Cardiac Stimulation Device And Method For Varying Pacing Parameters To Mimic Circadian Cycles ”, which is also incorporated by reference herein.
By using activity variance or minute ventilation, many of the problems associated with conventional circadian state detection techniques are overcome. However, room for improvement remains. In particular, minute ventilation and activity-based detection techniques can be adversely affected by frequent movement of the patient while asleep, as can occur with patients who are restless sleepers or with patients with labored breathing while asleep. Congestive heart failure (CHF) patients suffering from severe Cheyne-Stokes respiration often have quite labored breathing while asleep causing both elevated minute ventilation levels and activity levels. Hence, techniques relying only on minute ventilation and/or activity levels can erroneously conclude the patient is awake instead of asleep.
Accordingly, it would be desirable to provide an improved technique for detecting circadian states and it is to this end that aspects of the invention are generally directed. In particular, the invention is generally directed to exploiting blood carbon dioxide (CO2) parameters either alone or in combination with minute ventilation and activity levels for detecting circadian states. In this regard, it has been found that patients can tolerate a higher partial pressure of CO2 (pCO2) in the blood stream while asleep than while awake. Hence, average pCO2 levels are generally higher, on the average, while asleep than while awake. Although still higher levels can be achieved while exercising, patients with pacemakers or ICDs typically do not exercise often enough to elevate average waking pCO2 levels above average sleeping pCO2 levels. Moreover, it is the increasing concentration of pCO2 in the blood stream during the end tidal phase of the breathing cycle (also referred to herein as etCO2) that ultimately triggers inhalation. Since patients tolerate a higher concentration of pCO2 in the blood stream while asleep, etCO2 is slightly higher, again on the average, while sleep than while awake. In addition, it has been found that, on the average, the difference between the minimum and maximum pCO2 concentrations within individual breathing cycles (referred to herein as ΔcycleCO2) is greater while awake than while asleep. Hence, these and other blood CO2-based parameters can be used to distinguish between sleeping and waking states, i.e. to detect circadian states, so that pacing control parameters can be adjusted accordingly.
At least one technique has been developed for detecting blood CO2 levels using an implanted device. See U.S. Pat. No. 4,716,887 to Konig et al., entitled “Apparatus and Method for Adjusting Heart/Pacer Rate Relative to Cardiac PCO2 to Obtain a Required Cardiac Output”. With the technique of Konig et al., average pCO2 levels are detected and used in the adjustment of rate-responsive pacing rates under the assumption that higher pCO2 levels generally correspond to a higher exercise states, thus requiring higher pacing rates. In other words, the technique detects changes in pCO2 with time (ΔpCO2) and adjusts pacing rates based on ΔpCO2. Although the assumption that higher pCO2 levels generally correspond to a higher exercise states may be true while a patient is awake, this does not recognize the fact that average pCO2 levels are actually higher while asleep than while awake, at least for typical patients having pacemakers and ICDs. In any case, Konig et al. does not provide for the detection of circadian states based on blood CO2 parameters but only for rate responsive pacing.